Preforms are known with walls of flexible material which can be collapsed on itself to give a radial dimension which is much smaller than the diameter of the well or conduit to be cased in the event of local repair of a well or casing. The preform is introduced into the well or conduit in its collapsed state and once it is in the desired position, it is deployed by radial outward deformation, for example by introducing an inflating fluid such as a gas or liquid inside the preform. The applied pressure causes it to mould to the shape of the well or conduit. Such preforms normally comprise a thermosetting resin. In general, the resin forms part of the constituting wall of the preform and is very often introduced by impregnating the fibres forming the wall. Hardening the resin by thermosetting may be brought about by the temperature in the well, but normally it is caused either by introducing a hot fluid inside the preform or by a Joule effect using suitable electrical resistors located in the preform, for example electric wires forming part of the frame of the preform wall, impregnated with thermosetting resin. The heat released by an exothermic reaction which is initiated inside the preform can also be used, usually by bringing the reactants into contact at the desired time.